A hippocampal interneuron associated with the mossy fiber system

Network properties of the hippocampus emerge from the interaction of principal cells and a heterogeneous population of interneurons expressing gamma-aminobutyric acid (GABA). To understand these interactions, the synaptic connections of different types of interneurons need to be elucidated. Here we describe a type of inhibitory interneuron of the hippocampal CA3 region that has an axon coaligned with the mossy fibers. Whole-cell patch-clamp recordings, in combination with intracellular biocytin filling, were made from nonpyramidal cells of the stratum lucidum under visual control. Mossy fiber-associated (MFA) interneurons generated brief action potentials followed by a prominent after-hyperpolarization. Subsequent visualization revealed an extensive axonal arbor which was preferentially located in the stratum lucidum of CA3 and often invaded the hilus. The dendrites of MFA interneurons were mainly located in the strata radiatum and oriens, suggesting that these cells are primarily activated by associational and commissural fibers. Electron microscopic analysis showed that axon terminals of MFA interneurons established symmetric synaptic contacts predominantly on proximal apical dendritic shafts, and to a lesser degree, on somata of pyramidal cells. Synaptic contacts were also found on GABAergic interneurons of the CA3 region and putative mossy cells of the hilus. Inhibitory postsynaptic currents (IPSCs) elicited by MFA interneurons in simultaneously recorded pyramidal cells had fast kinetics (half duration, 3.6 ms) and were blocked by the GABA(A) receptor antagonist bicuculline. Thus, MFA interneurons are GABAergic cells in a position to selectively suppress the mossy fiber input, an important requirement for the recall of memory traces from the CA3 network.     

GIRK1 immunoreactivity is present predominantly in dendrites, dendritic spines, and somata in the CA1 region of the hippocampus

Electron microscopic analysis of the CA1 region of the rat hippocampus revealed that specific immunoreactivity (IR) for a G protein-gated, inwardly rectifying potassium channel (GIRK1) was present exclusively in neurons and predominantly located in spiny dendrites of pyramidal cells. Within stratum lacunosum-moleculare and the superficial stratum radiatum, GIRK1-IR was often present immediately adjacent to asymmetric (excitatory-type) postsynaptic densities in dendritic spines. The subcellular localization of GIRK1-IR in the Golgi apparatus of pyramidal cell somata and in the plasma membrane of dendrites and dendritic spines confirms the hypothesis that GIRK1 is synthesized by pyramidal cells and transported to the more distal dendritic processes. G protein-coupled receptor activation of a dendritic potassium conductance would attenuate the propagation of excitatory synaptic inputs and thereby produce postsynaptic inhibition. Thus, these results show that the GIRK family of channels joins the list of voltage-sensitive channels now known to be expressed in dendritic spines.     

INAUGURAL ARTICLE by a Recently Elected Academy Member:Chronic stress alters synaptic terminal structure in hippocampus

Repeated psychosocial or restraint stress causes atrophy of apical dendrites in CA3 pyramidal neurons of the hippocampus, accompanied by specific cognitive deficits in spatial learning and memory. Excitatory amino acids mediate this atrophy together with adrenal steroids and the neurotransmitter serotonin. Because the mossy fibers from dentate granule neurons provide a major excitatory input to the CA3 proximal apical dendrites, we measured ultrastructural parameters associated with the mossy fiber–CA3 synapses in control and 21-day restraint-stressed rats in an effort to find additional morphological consequences of stress that could help elucidate the underlying anatomical as well as cellular and molecular mechanisms. Although mossy fiber terminals of control rats were packed with small, clear synaptic vesicles, terminals from stressed animals showed a marked rearrangement of vesicles, with more densely packed clusters localized in the vicinity of active zones. Moreover, compared with controls, restraint stress increased the area of the mossy fiber terminal occupied by mitochondrial profiles and consequently, a larger, localized energy-generating capacity. A single stress session did not produce these changes either immediately after or the next day following the restraint session. These findings provide a morphological marker of the effects of chronic stress on the hippocampus that points to possible underlying neuroanatomical as well as cellular and molecular mechanisms for the ability of repeated stress to cause structural changes within the hippocampus.     

Persistent increase of hippocampal presynaptic axon excitability after repetitive electrical stimulation: dependence on N-methyl-D-aspartate receptor activity, nitric-oxide synthase, and temperature.

The electrical excitability of Schaffer collateral axons and/or terminals was studied in hippocampal slices by monitoring single, CA3 pyramidal neurons activated antidromically from CA1 stratum radiatum. At 22 degrees C, weak, repetitive stimulation with as few as 10 impulses at 2 Hz led to a robust lowering of the antidromic activation threshold that lasted > 30 min. The effect was completely absent at 32 degrees C and was blocked by both the N-methyl-D-aspartate receptor antagonist, 2-amino-5-phosphonovalerate and the inhibitor of nitric-oxide synthase, L-nitro-arginine methyl ester. Such threshold lowering would alter the variance of synaptic responses from axons stimulated in the variable excitation region of their input-output functions. These results thus raise important doubts about the interpretation of experiments in which the so-called minimal-stimulation method has been used at reduced temperature to infer changes in quantal transmission during hippocampal long-term potentiation. In the present experiments, no changes were observed in the estimate of excitatory postsynaptic potential quantal content in long-term potentiation experiments at either temperature, which could not be accounted for by an artificial, temperature-dependent change in the responsiveness of presynaptic axons.     

Mossy fiber-evoked subthreshold responses induce timing-dependent plasticity at hippocampal CA3 recurrent synapses

Dentate granule cells exhibit exceptionally low levels of activity and rarely elicit action potentials in targeted CA3 pyramidal cells. It is thus unclear how such weak input from the granule cells sustains adequate levels of synaptic plasticity in the targeted CA3 network. We report that subthreshold potentials evoked by mossy fibers are sufficient to induce synaptic plasticity between CA3 pyramidal cells, thereby complementing the sparse action potential discharge. Repetitive pairing of a CA3–CA3 recurrent synaptic response with a subsequent subthreshold mossy fiber response induced long-term potentiation at CA3 recurrent synapses in rat hippocampus in vitro. Reversing the timing of the inputs induced long-term depression. The underlying mechanism depends on a passively conducted giant excitatory postsynaptic potential evoked by a mossy fiber that enhances NMDA receptor-mediated current at active CA3 recurrent synapses by relieving magnesium block. The resulting NMDA spike generates a supralinear depolarization that contributes to synaptic plasticity in hippocampal neuronal ensembles implicated in memory.The CA3 area of the hippocampus exhibits a distinctive, highly recurrent circuitry proposed to support autoassociative memory representation (1, 2). This prediction has been confirmed by experimental work demonstrating the pattern completion capabilities of CA3 networks (3), as well as their roles in the spatial tuning of CA1 pyramidal cells, in one-trial contextual learning (4) and in certain forms of memory consolidation (5). CA3 pyramidal cells receive, via the mossy fibers, information processed by granule cells important for both pattern separation (6, 7) and pattern completion functions (7). The faithful transmission of mossy fiber input appears to be ensured by giant synapses composed of presynaptic boutons with up to 45 release sites (8) that target massive spines, the thorny excrescences, on the apical dendrite of CA3 pyramidal cells. Thus, the mossy fiber synapse is often referred to as a detonator synapse (9). In fact, mossy fiber signaling is more compatible with a gatekeeper function than a high-throughput data relay. Although high-frequency bursts of action potentials in a hippocampal granule cell can discharge a targeted CA3 pyramidal cell, the majority of responses evoked by granule cells in CA3 pyramidal cells do not attain the firing threshold (10). Nevertheless, mossy fibers generate powerful signals evoking subthreshold responses that are much larger than typical synaptic events in the brain, with excitatory postsynaptic potentials (EPSPs) and excitatory postsynaptic currents (EPSCs) reaching amplitudes of 10 mV and 1 nA, respectively (11). Here we examined in rat slice cultures how EPSPs generated at mossy fiber synapses are processed in CA3 pyramidal cell dendrites, and evaluated whether subthreshold synaptic responses evoked by mossy fiber stimulation can act as instructive signals to induce plasticity at the pyramidal cell synapses forming the CA3 recurrent network.     

Age-Dependent Inhibition of Long-Term Potentiation by Ethanol in Immature Versus Mature Hippocampus

The goal of this study was to assess the effects of ethanol on the induction of long-term potentiation (LTP) in hippocampal slices from immature versus mature rats. Population excitatory postsynaptic potentials (pEPSPs) were recorded from stratum radiatum of area CA1 of hippocampal slices using electrical stimulation of the Schaffer collateral/commissural fiber pathway. The slices were prepared from rats aged 15 to 25 or from 70 to 100 days. Long-term potentiation (LTP) of the pEPSP slope was induced using a single, θ-burst stimulus train in the presence or absence of 60 mM ethanol. Under control conditions, the stimulus train induced LTP in slices from both immature and mature animals. However, the magnitude of LTP was greater in slices from immature rats. When ethanol was present during the stimulus train, the magnitude of LTP in slices from mature animals did not differ significantly from the magnitude of LTP in control slices. However, ethanol virtually blocked the induction of LTP in slices from immature animals. These results indicate that memory-related synaptic plasticity in the hippocampus is attenuated by ethanol to a greater degree in immature versus mature animals.     

Estradiol increases pre- and post-synaptic proteins in the CA1 region of the hippocampus in female rhesus macaques (Macaca mulatta)

The role of estrogen (E) in promoting learning and memory in females has been well studied in both rodent and primate models. In female rats, E increases dendritic spine number, synaptogenesis, and synaptic proteins in the CA1 region of the hippocampus, an area of the brain that mediates learning and memory. In the present study, we used radioimmunocytochemistry to examine whether E and progesterone were capable of modulating the levels of pre- and postsynaptic proteins in the CA1 region of the female nonhuman primate hippocampus. It was found that E increased syntaxin, synaptophysin (presynaptic), and spinophilin (postsynaptic) levels in the stratum oriens and radiatum of the CA1 region, whereas combined E and progesterone treatment decreased these synaptic proteins to the levels found in untreated, spayed controls. Furthermore, progesterone treatment alone significantly increased synaptophysin levels in the stratum oriens and radiatum of the CA1 region. The levels of these synaptic proteins were unaltered by hormone treatment in the dentate gyrus, suggesting that this steroid-induced plasticity is hippocampal region specific. As these synaptic proteins are important components of the synaptic apparatus and reliable markers of synaptogenesis, it appears that E-induced increases in cognitive function of higher order mammals may be mediated in part by the effect of E on hippocampal synaptogenesis and synaptic plasticity.     

Distinct short-term plasticity at two excitatory synapses in the hippocampus

A single mossy fiber input contains several release sites and is located on the proximal portion of the apical dendrite of CA3 neurons. It is, therefore, well suited to exert a strong influence on pyramidal cell excitability. Accordingly, the mossy fiber synapse has been referred to as a detonator or teacher synapse in autoassociative network models of the hippocampus. The very low firing rates of granule cells [Jung, M. W. & McNaughton, B. L. (1993) Hippocampus 3, 165–182], which give rise to the mossy fibers, raise the question of how the mossy fiber synapse temporally integrates synaptic activity. We have therefore addressed the frequency dependence of mossy fiber transmission and compared it to associational/commissural synapses in the CA3 region of the hippocampus. Paired pulse facilitation had a similar time course, but was 2-fold greater for mossy fiber synapses. Frequency facilitation, during which repetitive stimulation causes a reversible growth in synaptic transmission, was markedly different at the two synapses. At associational/commissural synapses facilitation occurred only at frequencies greater than once every 10 s and reached a magnitude of about 125% of control. At mossy fiber synapses, facilitation occurred at frequencies as low as once every 40 s and reached a magnitude of 6-fold. Frequency facilitation was dependent on a rise in intraterminal Ca²⁺ and activation of Ca²⁺/calmodulin-dependent kinase II, and was greatly reduced at synapses expressing mossy fiber long-term potentiation. These results indicate that the mossy fiber synapse is able to integrate granule cell spiking activity over a broad range of frequencies, and this dynamic range is substantially reduced by long-term potentiation.     

Pre- and postnatal propylthiouracil-induced hypothyroidism impairs synaptic transmission and plasticity in area CA1 of the neonatal rat hippocampus

Thyroid hormones are essential for neonatal brain development. It is well established that insufficiency of thyroid hormone during critical periods of development can impair cognitive functions. The mechanisms that underlie learning deficits in hypothyroid animals, however, are not well understood. As impairments in synaptic function are likely to contribute to cognitive deficits, the current study tested whether thyroid hormone insufficiency during development would alter quantitative characteristics of synaptic function in the hippocampus. Developing rats were exposed in utero and postnatally to 0, 3, or 10 ppm propylthiouracil (PTU), a thyroid hormone synthesis inhibitor, administered in the drinking water of dams from gestation d 6 until postnatal day (PN) 30. Excitatory postsynaptic potentials and population spikes were recorded from the stratum radiatum and the pyramidal cell layer, respectively, in area CA1 of hippocampal slices from offspring between PN21 and PN30. Baseline synaptic transmission was evaluated by comparing input-output relationships between groups. Paired-pulse facilitation, paired-pulse depression, long-term potentiation, and long-term depression were recorded to examine short- and long-term synaptic plasticity. PTU reduced thyroid hormones, reduced body weight gain, and delayed eye-opening in a dose-dependent manner. Excitatory synaptic transmission was increased by developmental exposure to PTU. Thyroid hormone insufficiency was also dose-dependently associated with a reduction paired-pulse facilitation and long-term potentiation of the excitatory postsynaptic potential and elimination of paired-pulse depression of the population spike. The results indicate that thyroid hormone insufficiency compromises the functional integrity of synaptic communication in area CA1 of developing rat hippocampus and suggest that these changes may contribute to learning deficits associated with developmental hypothyroidism.     

Mice deficient for prion protein exhibit normal neuronal excitability and synaptic transmission in the hippocampus.

We recorded in the CA1 region from hippocampal slices of prion protein (PrP) gene knockout mice to investigate whether the loss of the normal form of prion protein (PrPC) affects neuronal excitability as well as synaptic transmission in the central nervous system. No deficit in synaptic inhibition was found using field potential recordings because (i) responses induced by stimulation in stratum radiatum consisted of a single population spike in PrP gene knockout mice similar to that recorded from control mice and (ii) the plot of field excitatory postsynaptic potential slope versus the population spike amplitude showed no difference between the two groups of mice. Intracellular recordings also failed to detect any difference in cell excitability and the reversal potential for inhibitory postsynaptic potentials. Analysis of the kinetics of inhibitory postsynaptic current revealed no modification. Finally, we examined whether synaptic plasticity was altered and found no difference in long-term potentiation between control and PrP gene knockout mice. On the basis of our findings, we propose that the loss of the normal form of prion protein does not alter the physiology of the CA1 region of the hippocampus.     

Src, a molecular switch governing gain control of synaptic transmission mediated by N-methyl-D-aspartate receptors.

The N-methyl-D-aspartate (NMDA) receptor is a principal subtype of glutamate receptor mediating fast excitatory transmission at synapses in the dorsal horn of the spinal cord and other regions of the central nervous system. NMDA receptors are crucial for the lasting enhancement of synaptic transmission that occurs both physiologically and in pathological conditions such as chronic pain. Over the past several years, evidence has accumulated indicating that the activity of NMDA receptors is regulated by the protein tyrosine kinase, Src. Recently it has been discovered that, by means of up-regulating NMDA receptor function, activation of Src mediates the induction of the lasting enhancement of excitatory transmission known as long-term potentiation in the CA1 region of the hippocampus. Also, Src has been found to amplify the up-regulation of NMDA receptor function that is produced by raising the intracellular concentration of sodium. Sodium concentration increases in neuronal dendrites during high levels of firing activity, which is precisely when Src becomes activated. Therefore, we propose that the boost in NMDA receptor function produced by the coincidence of activating Src and raising intracellular sodium may be important in physiological and pathophysiological enhancement of excitatory transmission in the dorsal horn of the spinal cord and elsewhere in the central nervous system.     

Gap junctions on hippocampal mossy fiber axons demonstrated by thin-section electron microscopy and freeze fracture replica immunogold labeling

Gap junctions have been postulated to exist between the axons of excitatory cortical neurons based on electrophysiological, modeling, and dye-coupling data. Here, we provide ultrastructural evidence for axoaxonic gap junctions in dentate granule cells. Using combined confocal laser scanning microscopy, thin-section transmission electron microscopy, and grid-mapped freeze-fracture replica immunogold labeling, 10 close appositions revealing axoaxonic gap junctions ( approximately 30-70 nm in diameter) were found between pairs of mossy fiber axons ( approximately 100-200 nm in diameter) in the stratum lucidum of the CA3b field of the rat ventral hippocampus, and one axonal gap junction ( approximately 100 connexons) was found on a mossy fiber axon in the CA3c field of the rat dorsal hippocampus. Immunogold labeling with two sizes of gold beads revealed that connexin36 was present in that axonal gap junction. These ultrastructural data support computer modeling and in vitro electrophysiological data suggesting that axoaxonic gap junctions play an important role in the generation of very fast (>70 Hz) network oscillations and in the hypersynchronous electrical activity of epilepsy.     

Reconstitution of the hippocampal mossy fiber and associational-commissural pathways in a novel dissociated cell culture system.

Synapses of the hippocampal mossy fiber pathway exhibit several characteristic features, including a unique form of long-term potentiation that does not require activation of the N-methyl-D-aspartate receptor by glutamate, a complex postsynaptic architecture, and sprouting in response to seizures. However, these connections have proven difficult to study in hippocampal slices because of their relative paucity (<0.4%) compared to commissural-collateral synapses. To overcome this problem, we have developed a novel dissociated cell culture system in which we have enriched mossy fiber synapses by increasing the ratio of granule-to-pyramidal cells. As in vivo, mossy fiber connections are composed of large dynorphin A-positive varicosities contacting complex spines (but without a restricted localization). The elementary synaptic connections are glutamatergic, inhibited by dynorphin A, and exhibit N-methyl-D-aspartate-independent long-term potentiation. Thus, the simplicity and experimental accessibility of this enriched in vitro mossy fiber pathway provides a new perspective for studying nonassociative plasticity in the mammalian central nervous system.     

A presynaptic locus for long-term potentiation of elementary synaptic transmission at mossy fiber synapses in culture.

The complex circuitry of the CA3 region and the abundance of collateral connections has made it difficult to study the mossy fiber pathway in hippocampal slices and therefore to establish the site of expression of long-term potentiation at these synapses. Using a novel cell culture system, we have produced long-term potentiation of the elementary synaptic connections on single CA3 pyramidal neurons following tetanic stimulation of individual dentate gyrus granule cells. As is the case for the hippocampal slice, this potentiation was independent of N-methyl-D-aspartate receptor activation, was simulated by application of forskolin, and its induction did not require any modulatory input. The increase in synaptic strength was accompanied by a reduction in the number of failures of transmission and by an increase in the coefficient of variation of the responses and was prevented by presynaptic injection of an inhibitor of protein kinase A. These findings show that mossy fiber long-term potentiation has a presynaptic locus and that its expression is dependent on protein kinase A.     

Dendritic K+ channels contribute to spike-timing dependent long-term potentiation in hippocampal pyramidal neurons

We investigated the role of A-type K(+) channels for the induction of long-term potentiation (LTP) of Schaffer collateral inputs to hippocampal CA1 pyramidal neurons. When low-amplitude excitatory postsynaptic potentials (EPSPs) were paired with two postsynaptic action potentials in a theta-burst pattern, N-methyl-d-aspartate (NMDA)-receptor-dependent LTP was induced. The amplitudes of the back-propagating action potentials were boosted in the dendrites only when they were coincident with the EPSPs. Mitogen-activated protein kinase (MAPK) inhibitors PD 098059 or U0126 shifted the activation of dendritic K(+) channels to more hyperpolarized potentials, reduced the boosting of dendritic action potentials by EPSPs, and suppressed the induction of LTP. These results support the hypothesis that dendritic K(+) channels and the boosting of back-propagating action potentials contribute to the induction of LTP in CA1 neurons.     

Inhibition of the production and maintenance of long-term potentiation in rat hippocampal slices by a monoclonal antibody

Monoclonal antibodies generated to 5-day-old postnatal rat dentate gyri were tested for their effects on long-term potentiation (LTP) in rat hippocampal slices. One antibody, B6E11, was found to block the production of LTP and to suppress established LTP in both area CA1 of the hippocampus and the dentate gyrus. In CA1, B6E11 was effective only when applied to the apical dendrites synapsing with the potentiating input. The production of LTP could not be blocked if B6E11 was applied to the cell bodies or to the basal dendrites of CA1. In field CA1, B6E11 had no effect on the production of short-term potentiation. Another monoclonal antibody from the same panel as B6E11, of the same immunoglobulin class, which also binds to hippocampal neurons similarly to B6E11, had no effect on LTP production. These results supply evidence that B6E11 modulates LTP by an interaction with a specific cell-surface protein associated with the dendrites of neurons in both dentate gyrus and CA1 regions.     

Maternal hypothyroxinemia impairs spatial learning and synaptic nature and function in the offspring

Neurological deficits in the offspring caused by human maternal hypothyroxinemia are thought to be irreversible. To understand the mechanism responsible for these neurological alterations, we induced maternal hypothyroxinemia in pregnant rats. Behavior and synapse function were evaluated in the offspring of thyroid hormone-deficient rats. Our data indicate that, when compared with controls, hypothyroxinemic mothers bear litters that, in adulthood, show prolonged latencies during the learning process in the water maze test. Impaired learning capacity caused by hypothyroxinemia was consistent with cellular and molecular alterations, including: 1) lack of increase of phosphorylated c-fos on the second day of the water maze test; 2) impaired induction of long-term potentiation in response to theta-burst stimulation to the Schaffer collateral pathway in the area 1 of the hippocampus Ammon's horn stratum radiatum, despite normal responses for input/output experiments; 3) increase of postsynaptic density protein 95 (PSD-95), N-methyl-D-aspartic acid receptor subunit 1, and tyrosine receptor kinase B levels in brain extracts; and 4) significant increase of PSD-95 at the PSDs and failure of this molecule to colocalize with N-methyl-D-aspartic acid receptor subunit 1, as it was shown by control rats. Our findings suggest that maternal hypothyroxinemia is a harmful condition for the offspring that can affect key molecular components for synaptic function and spatial learning.     

Acamprosate (Calcium Acetylhomotaurinate) Enhances the N-Methyl-d-Aspartate Component of Excitatory Neurotransmission in Rat Hippocampal CA1 Neurons In Vitro

The taurinate analog acamprosate (calcium acetylhomotaurinate) has received considerable attention in Europe for its ability to prevent relapse in abstained alcoholics. To determine the mechanism of acamprosate actions in the CNS, we superfused acamprosate onto rat hippocampal CA1 pyramidal neurons using an in vitro slice preparation. In current- and voltage-clamp recordings, acamprosate (100 to 1000 μM) superfusion had little effect on resting membrane potential or input slope resistance. Acamprosate had no effect on Ca²⁺-dependent action potentials when tetrodotoxin was used to block Na⁺ spikes. In whole-cell voltage-clamp recordings, and in the presence of tetraethylammonium and Cs⁺ to block K⁺ channels, acamprosate had little effect on a Cd²⁺-sensitive inward current likely to be a high voltage-activated Ca²⁺ current. However, in both current- and voltage-clamp recordings, acamprosate significantly increased the N-methyl-d -aspartate (NMDA) component of excitatory postsynaptic potentials evoked by stimulation of Schaffer collaterals in the stratum radiatum, in the presence of the selective non-NMDA (R.S)-α-amino-3-hydroxy-5-methylisoxazole-4-proprionic acid kainate) glutamate receptor antagonist 6-cyano-7-nitro-quinoxaline-2,3-dione and the GABA_A receptor antagonist bicuculline. Acamprosate had inconsistent or no effects on the stratum radiatum-evoked non-NMDA component of the excitatory postsynaptic potentials, in the presence of bicuculline and the NMDA antagonist dl -2-amino-5-phosphonovalerate. Acamprosate, on average, had little effect on the late inhibitory postsynaptic potentials thought to be mediated by GABA_B receptors. In the presence of tetrodotoxin to block synaptic transmission, acamprosate dramatically increased inward current responses in most CA1 neurons to exogenous NMDA applied by pressure or superfusion, with reversal on washout of acamprosate. These data suggest that acamprosate may act postsynaptically to increase the NMDA component of excitatory transmission to hippocampal CA1 pyramidal neurons. Considering the known interaction of ethanol with NMDA receptors, this acamprosate modulation of NMDA receptor-mediated neurotransmission could provide a mechanism of action underlying the clinical efficacy of acamprosate.     

Perinatal hypothyroidism decreases hippocampal mossy fiber zinc density in rats.

The effect of perinatal hypothyroidism on hippocampal mossy fiber zinc density was examined in rats. Timed pregnant Sprague-Dawley rat dams were given water containing either 0.02% propylthiouracil (PTU) or vehicle from gestational day 18 until their litters were weaned on postnatal day 31. Hippocampal mossy fiber zinc density was reduced by 75% in both the dorsal and ventral hippocampal formation CA3 stratum lucidum region of 31-day-old PTU-treated rats compared to untreated controls. Perinatal hypothyroidism did not alter hippocampal tissue zinc concentration, indicating that the PTU-induced reduction in mossy fiber zinc was not a consequence of reduced hippocampal zinc concentration. At 120 days of age, 3 months after discontinuation of PTU treatment, hippocampal mossy fiber zinc density remained significantly reduced by 33-45% in PTU-treated rats compared to control. These data indicate that perinatal hypothyroidism causes a long-lasting reduction in hippocampal mossy fiber zinc density.     

Glutamate and γ-aminobutyric acid mediate a heterosynaptic depression at mossy fiber synapses in the hippocampus

Mossy fiber synapses form the major excitatory input into the autoassociative network of pyramidal cells in the CA3 area of the hippocampus. Here we demonstrate that at the mossy fiber synapses, glutamate and γ-aminobutyric acid (GABA) act as autaptic and heterosynaptic presynaptic inhibitory transmitters through metabotropic glutamate receptors (mGluRs) and GABA_B receptors, respectively. Both GABA_B receptors and mGluRs are activated through spillover from adjacent synapses. We demonstrate that glutamate spillover caused by brief tetanic activation of mossy fiber terminals remains intact at physiological temperatures. The activation of GABA_B receptors increased the threshold for mossy fiber long-term potentiation (LTP), whereas activation of mGluRs did not have such an effect. We speculate that this heterosynaptic depression provides the mossy fiber synapses with a mechanism to efficiently shape input patterns into CA3, increasing the sparseness of the mossy fiber signal and enhancing the capacity and performance of the CA3 associative network. The increase in LTP threshold through activation of presynaptic inhibitory receptors imparts a piesynoptic associative nature to mossy fiber LTP.